Recently Cannot send email from iphone or ipad

Recently Cannot send email from iphone or ipad

After years of being able to do so, yesterday I found I cannot send AT&T email from my iphone or ipad.

The only thing different was my receipt of a notice that my account had been accessed from an unknown device in Bulgaria. I changed my password right away and saw that 2 emails I did not write had been sent from my account.

Now outgoing messages get a failed to send to server message and stay in my outbox when sent from my iphone or ipad, but not when sent from my PC.

Can anyone help me fix this so I can send AT&T email from my iphone or ipad again?